Musnad Ahmad 32:171

Abu Mu'awiyah narrated to us, Mas-haj al-Dabbi narrated to us, he said: I heard Anas ibn Malik saying: "When we were with the Prophet, may Allah bless him and grant him peace, on a journey, and we would say: 'The sun has declined' or 'It has not declined,' he would pray Dhuhr, then depart."